Small for Gestational Age (SGA) is a term for a baby who is smaller than expected for the number of weeks of pregnancy. These infants have a birth weight that falls below the 10th percentile for their gestational age, meaning they are smaller than 90 percent of babies of the same sex and age. This measurement classifies a baby’s size at birth and does not, by itself, explain the reason the baby is small.
It is helpful to distinguish SGA from Intrauterine Growth Restriction (IUGR). While SGA is a size-based classification, IUGR is a clinical diagnosis that a fetus is not growing at a normal rate in the womb due to a specific problem. A baby can be SGA simply because of genetics if their parents are small in stature. Conversely, a baby might experience IUGR and show signs of malnutrition but still have a birth weight above the 10th percentile, meaning they would not be classified as SGA.
Causes and Risk Factors
The reasons a baby might be small for gestational age are varied and can be grouped into maternal, placental, or fetal factors. Maternal health plays a significant part, with conditions like chronic high blood pressure, advanced diabetes, or kidney disease being risk factors. Lifestyle choices during pregnancy, including smoking, alcohol consumption, and poor nutrition, can also restrict a baby’s growth. In many situations, the exact reason for a baby being SGA is not identified.
The placenta’s function is another determinant of fetal growth. If there is decreased blood flow in the uterus and placenta, the fetus may not receive the oxygen and nutrients needed to grow properly. Placental issues, such as placenta previa, where the placenta attaches low in the uterus, or placental abruption, can impede this supply line. Infections in the tissues surrounding the fetus can also contribute to growth problems.
Fetal factors also account for a portion of SGA cases. A pregnancy with multiple babies, such as twins or triplets, often results in each baby being smaller. Infections acquired by the fetus during pregnancy can slow its growth. Genetic or chromosomal abnormalities can also be an underlying cause for a baby being smaller than average.
Diagnosis and In-Utero Monitoring
Identifying a baby as small for gestational age often begins with a routine measurement during prenatal appointments called the fundal height. This is the distance from the top of the mother’s pubic bone to the top of the uterus. If this measurement is smaller than expected for the stage of pregnancy, it may prompt further investigation to assess the baby’s growth.
Ultrasound is a primary tool for diagnosing and monitoring a fetus suspected of being SGA. A detailed ultrasound, known as fetal biometry, measures the baby’s head circumference, abdominal circumference, and femur bone length. These measurements are then used to estimate the fetus’s weight and compare it to standard growth charts. This helps determine if the baby’s size is below the 10th percentile.
When SGA is suspected, more specialized monitoring is often implemented. A Doppler ultrasound is a test that measures blood flow in the umbilical cord and in vessels within the baby’s brain. This assessment helps determine if the placenta is delivering adequate blood, oxygen, and nutrients. Prenatal visits may become more frequent and include other surveillance, such as nonstress tests to monitor the fetal heart rate, and biophysical profiles, which assess heart rate, breathing, movement, muscle tone, and amniotic fluid levels.
Postnatal Care and Immediate Considerations
Newborns who are small for gestational age require close observation in the nursery. One of the primary concerns is hypoglycemia, or low blood sugar. Because these babies have less body fat and smaller livers, they have lower stores of glycogen, the body’s stored form of glucose. This can lead to their blood sugar levels dropping quickly, so regular monitoring is standard practice.
Maintaining body temperature is another challenge for SGA infants. With less body fat to act as insulation, they can lose heat rapidly and are more susceptible to hypothermia. Incubators or radiant warmers are often used to provide a stable, warm environment. This helps the baby conserve energy that would otherwise be used to stay warm, allowing more calories to be directed toward growth.
Feeding support is also a focus of postnatal care. SGA babies need sufficient caloric intake to begin “catch-up” growth, but may have difficulty with feeding due to their small size or low energy levels. Healthcare providers may support breastfeeding or recommend supplemental feedings with expressed breast milk or a high-calorie formula. In some cases, a feeding tube might be temporarily needed. Another condition that is monitored is polycythemia, an excess of red blood cells, which can occur as the body’s response to a low-oxygen environment in the womb.
Long-Term Growth and Development
The majority of babies born small for gestational age experience a period of accelerated growth after birth, often called “catch-up growth.” This rapid growth phase typically occurs within the first two years of life. It allows most of these children to reach a height and weight within the normal range, comparable to their peers.
While catch-up growth is common, a small percentage of children born SGA may not fully catch up and may remain shorter than average. The long-term growth pattern can depend on the underlying cause of the SGA condition, such as genetics or an in-utero growth problem. Consistent monitoring of growth by a pediatrician is important to track a child’s progress.
Research has pointed to an association between being born SGA and a higher risk for developing certain health conditions in adulthood. These can include aspects of metabolic syndrome, such as high blood pressure, obesity, and type 2 diabetes. This highlights the value of establishing healthy lifestyle habits, including a balanced diet and regular physical activity, from an early age.